1. Field of the Invention
The present invention relates to an optical I/O module such as an optical add-drop multiplexer which drops a given wavelength component from signal light having a plurality of wavelength components and adds a given wavelength component to the signal light, and a light-reflecting device suitable for the optical I/O module.
2. Related Background Art
In the field of wavelength-division multiplexing (WDM) transmission, attention has been drawn to the add-drop multiplexer (ADM), as an optical I/O module, which drops a given wavelength component from WDM signal light having a plurality of wavelength components and adds a given wavelength component to the signal light. An example of the ADM is disclosed, for example, in the publication titled "Optical ADM Experiments Using Fiber Grating and Limiting Factors thereof," the Institute of Electronics, Information and Communication Engineers, General Conference 1996, SB-11-7, p. 747-748.
As shown in FIG. 1, the ADM disclosed in the above-mentioned publication is disposed in an optical fiber main line MF (transmission line) in a communication network via an input connector C.sub.in and an output connector C.sub.out. In this ADM, a part of the transmission line is constituted by two pieces of optical circulators OC.sub.1, OC.sub.2 and a transmission type optical fiber grating FG. The optical fiber grating FG has such a demultiplexing characteristic that it reflects, of WDM signal light including a plurality of wavelength components .lambda..sub.1 to .lambda..sub.n, only a predetermined wavelength component .lambda..sub.i, and transmits therethrough the rest of wavelength components .lambda..sub.1 to .lambda..sub.i-1, .lambda..sub.i+1 to .lambda..sub.n.
When the WDM signal light including a plurality of wavelength components .lambda..sub.1 to .lambda..sub.n enters the AMD from the upstream side of the main line MF via the connector C.sub.in, the WDM signal light propagates from a port P.sub.1 of the optical circulator OC.sub.1 to a port P.sub.2 thereof, thereby reaching the optical fiber grating FG. The wavelength components .lambda..sub.1 to .lambda..sub.i-1, .lambda..sub.i+1 to .lambda..sub.n, transmitted through the optical fiber grating FG successively pass through ports P.sub.4 and P.sub.5 of the optical circulator OC.sub.2, so as to be emitted to the downstream side of the optical fiber main line MF via the connector C.sub.out. The wavelength component .lambda..sub.i reflected by the optical fiber grating FG is dropped to the dropping port P.sub.3.
On the other hand, a wavelength component .lambda..sub.I at the same wavelength as the wavelength component .lambda..sub.i is added to the adding port P.sub.6 of the optical circulator OC.sub.2. Thus added wavelength component .lambda..sub.I propagates to the port P.sub.4 by way of the port P.sub.6, and then is reflected by the optical fiber grating FG. Thus reflected wavelength component .lambda..sub.I successively passes through the ports P.sub.4 and P.sub.5, so as to be emitted to the downstream side of the optical fiber main line MF via the connector C.sub.out as WDM signal light including the wavelength component .lambda..sub.I in addition to the wavelength components .lambda..sub.1 to .lambda..sub.i-1, .lambda..sub.i+1 to .lambda..sub.n.
Thus, the conventional ADM realizes the drop and addition of a given wavelength component by serially connecting the two optical circulators OC.sub.1, OC.sub.2 and the transmission type optical fiber grating FG in the optical fiber main line MF (thereby constituting a part of the transmission line).